Many bacterial pathogens important to human health evade the immune system by living within white blood cells. Salmonella enterica, a species of gram-negative bacteria that includes the causative agent of human typhoid fever, resides within a class of white blood cells called macrophages. We have demonstrated that in mice, S. enterica subspecies Typhimurium (Salmonella) resides and replicates within hemophagocytic macrophages (HMs), which are macrophages that have engulfed erythrocytes, platelets, leukocytes and their precursor cells. Our long-term goal is to determine how Salmonella and HMs interact to cause disease. Mice infected with Salmonella are a natural host-pathogen model system encountered in the wild. Salmonella causes an acute infection in mice that typically resolves into a chronic infection, and the disease course resembles that of typhoid fever. The bacteria colonize the spleen, liver, and the lymph nodes that drain the intestine. We demonstrated the presence of HMs within the spleen, liver and bone marrow of Salmonella - infected mice and identified HMs containing Salmonella as late as eight weeks post-infection in the liver, when persistent infection has been established. In Preliminary Studies we developed a flow cytometric assay to identify and separate HMs from the spleen. This novel methodology along with established approaches enables new exploration of the role of HMs in disease. The objectives of the current proposal are to 1) Determine the immunological requirements for hemophagocytosis and the effect of HM accumulation on the course of murine typhoid fever, 2) Establish whether HMs formed in response to Salmonella infection in vivo or in culture become anti-inflammatory and whether anti-inflammatory macrophages are permissive for bacterial replication, and 3) Identify regulatory pathways within HMs needed to make them permissive for Salmonella replication. Completion of these Aims has the potential to elucidate whether hemophagocytosis benefits the host as well as Salmonella. In addition, the information we acquire has potential use in the development of treatments that modulate hemophagocytosis and influence the course of inflammation in infectious and noninfectious circumstances.